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Olduino/Z first Board Layout

December 9, 2015

This isn’t finished and i’m sure there are errors but it went together suprisingly easily.

The biggest choices to make are how to connect to the arduino headers.

  • Four of the arduino pins are dedicated to SPI communications and IC1 (TP3465V) that provides that.
  • Seven of the Z80’s output bits are wired to arduino positions D3 through D9 (the eigth is serial out).
  • Five of the Z80’s input bits are wired to what would be analog input positions on an arduino. Analog input pins are also used as digital on an arduino and I found this worked well on the 1802 version.
  • Two of the other inputs are used for serial and timer and the other one is spare for the moment.
  • The SPI chip also has four bidirectional I/O pins meant for SPI select lines. I’ve wired one to what would normally be D10 on an arduino for that purpose and two others to D2 and analog 5 just for fun.

The select logic for the SPI chip at locations 80-8F is handled by a nand gate.

There is an attiny85 avr chip dedicated to a millisecond clock. All it does is issue a brief (25us) negative going pulse every millisecond which interrupts the Z80. There are three schottky diodes for wire-ORing the reset and interrupt lines.

The board layout is just my raw first try. The left half where the avr and diodes are would be covered with a chunk of solderless protoboard. The Eagle autorouter takes it with no complaint but i need to try hand drawing some of the connections to see if there’s any big gains to be made. If I can I’ll leave some IC footprints or a general solder grid free for my inevitable screw-ups.

I could reduce the board width a bit if I rotated the arduino rails 90 degrees and put them inside the CPU board connectors. I have a feeling though that I tried this on one of the olduino/1802 adapters and it made attaching shields awkward – I’ll look ion my failure drawer.

I’m planning to finish the eagle layout in the next few days so i can get it produced and assembled before the end of January. That way i can play with the software over the winter when i’ll be away from my workbench.

15-12-09 brd 5a15-12-09 sch 5a

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